208 SCIENCE AND FRUIT GROWING 



there was a much greater discrepancy between the results of 

 the examination of the eggs and the hatchings, for the former, 

 in the instances selected, had indicated a mortality of only 5 per 

 cent., whereas the hatchings showed a mortality of as much as 

 96 per cent. This high value, however, was in a sense fictitious, 

 for on examining the contents of the scales, it was found that 

 the eggs had not really been killed, but that the insects had 

 hatched out, and had been unable to emerge -from the scales, 

 owing to these having become glued to the bark (see p. 202). 



-Though the ultimate effect of simple alkalis may thus be as good 

 as that of alkaline emulsions, a preference should certainly be 

 given to the latter, for it is evidently better to kill the eggs, 

 than to trust to the glueing of the scales to the bark, as this 

 ma}/ be much influenced by the weather conditions, the date 

 of the spraying, and the condition of the wax forming the scales ; 

 for it was noticeable that in many cases the scales had not become 

 glued down, and that then there was a free hatching of eggs 

 (VI, 114). It seems probable that, where aqueous solutions of 

 alkalis of not more than 2 per cent, strength are used, the alkali 

 never penetrates at all to the eggs, and that, the great enhance- 

 ment of effect obtained by the addition of soap, paraffin, etc., to 

 the alkali is due to their enabling the liquid to soak in between 

 the scale and the bark, and thus reach the eggs. This also 

 happens when the strength of the alkali is sufficient to destroy 

 the substance of scale, as when 2-5 to 3 per cent, solutions are 

 used. 



In practical spraying the application of the wash is neces- 

 sarily less perfect than when it is applied, as it was in these 

 experiments, by means of a paint brush, and a 90 per cent, 

 efficiency is the maximum which might be anticipated with the 

 most effectual wash. If, as the Woburn results indicated, 

 20 per cent, of the eggs in the untreated scale hatch out, and there 

 are on the average 40 eggs under each scale, a 90 per cent, 

 efficiency would reduce the number of insects hatching to about 

 one per scale, and, as there must be a considerable mortality 

 amongst the insects after they are hatched, the pest would, at this 

 rate, very soon become exterminated. This is in accordance with 

 the practical results obtained at Woburn (see p. 205). 



For a more rapid riddance of this pest, winter treatment of 

 the trees may be supplemented with summer treatment, as is 

 now generally recommended in the case of the San Jose scale : 

 for this purpose a weak paraffin emulsion, such as that men- 

 tioned on p. 160, may be used, spraying it on to the trees, or 



